Movie clip node as mask
Instead of using a movie clip node use an Image Sequence node (you can also load videos in it)

Then you will have control of how many frames to use, at what point in time and offset.

To hold the last frame just untick the Cycle box



See these related links: 

https://blender.stackexchange.com/a/9123/1853

https://blender.stackexchange.com/a/8985/1853